1. Field of the Invention
The present invention relates to communications, and, in particular, to loop testing for communication systems.
2. Description of the Related Art
General Description of an Integrated Digital Loop Carrier Architecture
The generic requirements that describe the overall system functions, performance, and operation for a digital loop carrier (DLC) system when integrated into a local digital switch (LDS) are given in Bellcore Generic Requirements, GR-303-Core, "Integrated Digital Loop Carrier System Generic Requirements, Objectives, and Interface," Issue 1, September 1995, plus Revision 2, December 1996, (A Module of TSGR, FR-NWT-000440). These requirements specify one particular way to interface a DLC remote digital terminal (RDT) with an LDS. The requirements for such integrated digital loop carrier (IDLC) systems are expressed in terms of functions that the combined remote digital terminal and local digital switch shall have when connected together. The interface defined by the cited Bellcore documents is commonly referred to as a TR-303 interface.
FIG. 1 shows a block diagram of an integrated digital loop carrier system 100 conforming to the TR-303 interface requirements. IDLC system 100 comprises an integrated digital terminal (IDT) 104, which is part of a local digital switch 102, and a remote digital terminal 106. The digital transmission facilities (DTF) 105 carry communication signals between local digital switch 102 and remote digital terminal 106. The DTF can be either metallic or fiber. The TR-303 interface requirements specify that the IDLC system shall have at least one test access path (TAP) 108, which is a metallic or a metallic-like connection from the integrated digital terminal to the remote digital terminal used for subscriber loop testing. A test access path provides DC continuity from the local digital switch to the loop 109 to the subscriber 110. The terms "loop" and "line" are used interchangeably in this specification. It may be, but is not necessarily, implemented as a copper pair. At the remote digital terminal, an individual subscriber loop (e.g., 109) can be switched to the test access path for testing. This arrangement provides a test path all the way from the local digital switch to the subscriber 110.
Remote digital terminals, such as RDT 106 in FIG. 1, are deployed either in the outside plant (e.g., in cabinets, controlled environment vaults (CEVs), or telephone huts) or on customer premises. The specification for a remote digital terminal allows it to support a large number of lines (up to 2048 subscriber lines that can vary in length over a wide range). Thus, the cost associated with a test access path is shared across this large number of lines. Because test access paths are costly to implement and maintain, some local digital switches support only one test access path per TR-303 interface (e.g., 112 in FIG. 1). Because the loop-testing algorithms used by the loop testing system 103 to determine if a fault exists and, if one does exist, where it is located (i.e., distance to fault), have to accommodate a wide range of line lengths, they are very complex.
Loop Testing in an IDLC System
In an integrated digital loop carrier system, such as IDLC system 100 of FIG. 1, loop testing is implemented under the control of a loop testing system, such as loop testing system 103 of FIG. 1. A loop testing system typically comprises hardware, software, and databases designed and used to control loop testing of individual subscriber loops, such as loops 109 of FIG. 1. Loop testing in an integrated digital loop carrier system is implemented as follows:
(1) The loop testing system, using the subscriber telephone number as a test address, sends a test request to the local digital switch. This initiates the following sequence. PA1 (2) Optional test of the test path between the local digital switch and the remote digital terminal. PA1 (3) The local digital switch sends a message to the remote digital terminal to begin the setup sequence for the test path. This alerts the remote digital terminal of a test request. PA1 (4) When the local digital switch sends a message to the remote digital terminal to connect the test access path, the remote digital terminal connects the test access path to the channel unit (CU) under test (114 in FIG. 1), and instructs the channel unit to connect the test access path to the subscriber loop 109. This completes the test path from the loop testing system to the subscriber. PA1 (5) The loop testing system then runs its loop tests. PA1 (6) After completion of the loop tests, the local digital switch sends a message to the remote digital terminal to disconnect the test access path. PA1 (7) The remote digital terminal then releases the test access path connection to the channel unit and instructs the channel unit to disconnect the subscriber loop from the test access path and to reconnect the subscriber loop to the channel unit's line circuit. This completes the loop test session.
The local digital switch runs a diode test to determine the integrity of the test path between it and the remote digital terminal. In response to the request for the diode test from the local digital switch, the remote digital terminal terminates the test access path with a diode/resistor circuit. The local digital switch recognizes this termination, thus determining that the test access path connection to the remote digital terminal is intact. PA2 The local digital switch then sends a message to the remote digital terminal to release the diode/resistor termination from the test access path, thus completing the diode test.
Fiber-to-the Curb Architecture
FIG. 2 shows a block diagram of a fiber-to-the-curb (FTTC) system 200. In an FTTC system, the remote digital terminal (e.g., RDT 106 in FIG. 1) is replaced by a host digital terminal (HDT) 216. The host digital terminal can be deployed either in the central office (CO) or in the outside plant (e.g., in cabinets, controlled environment vaults, or telephone huts). Subtending from the host digital terminal and connected to it by optical fibers 217 are one or more optical network units (ONUs) 206, where at least some of the ONUs may be separated from one another by significant distances (i.e., non-collocated). An optical network unit houses the various channel units 214 that provide service to the subscribers 210. The digital transmission facilities (DTF) 205, which can be either metallic or fiber, carry communication signals between the local digital switch 202 and the host digital terminal 216. When the host digital terminal is located in the central office, the digital transmission facilities are usually metallic. When the host digital terminal is located in the outside plant, the digital transmission facilities are fiber and the host digital terminal extends the fiber connection from the local digital switch all the way to the optical network unit. The fiber-to-the-curb architecture allows the ONU electronics (i.e., the subscriber interface) to be placed closer to the subscribers. The optical network units are generally placed 500 feet or less from the subscriber and support a small number (e.g., 12, 24, or 48) of subscriber loops 209. This FTTC architecture eliminates long copper subscriber loops and the problems associated with them. However, it creates a problem for loop testing.
Typical integrated digital terminals, such as IDT 204 of FIG. 2, support a limited number (e.g., three) of test access paths 208, with some IDTs supporting only one TAP. Furthermore, an integrated digital terminal may be unaware of the existence of the host digital terminal and the multiple non-collocated optical network units in an FTTC architecture. The integrated digital terminal "thinks" the host digital terminal is a remote digital terminal. As such, loop testing cannot be performed using the IDLC model of FIG. 1. Even if the integrated digital terminal could support a sufficient number of test access paths, it may be cost prohibitive to run a test access path to each non-collocated optical network unit due to the small line size supported there. To address these problems, Lucent Technologies developed a method of loop testing for FTTC systems that is transparent to the local digital switch and to the loop testing system of the IDLC model. This method takes advantage of the fact that the subscriber loops 209 in an FTTC system are relatively short and therefore require only a basic set of simplified fault detection tests. This set of tests is implemented by a test head 218 located in the optical network unit that uses simple test algorithms and reports the results of the tests to the host digital terminal. The host digital terminal presents a set of resistive signatures to the test access path, each of which corresponds to a pass/fail result of a specific test.